1. Field of the Invention
The invention relates to a method for detection of oxidizable and/or reducible gases in the air for the purpose of controlling devices for ventilation in buildings or vehicles and for the purpose of surveillance of combustion processes or of waste gas catalyst plants, by employing at least one heated and current-passing sensor, furnished with contact electrodes and made of a metallic sensor material, as well as by employing an electrical evaluation circuit, wherein the sensor is passed through by an alternating current, which alternating current either comprises at least two alternating currents of different frequencies or is switched between at least two frequencies, and wherein, on the one hand, the change of the capacitances between the sensor material and the contact electrodes is evaluated by the evaluation circuit as an indicator of a presence of reduceable gases and, on the other hand, the change of the capacitances within the mass of the sensor material (sensitive material) is evaluated as an indicator of the presence of oxidizable gases, as well as to a device for the performance of the method.
2. Brief Description of the Background of the Invention Including Prior Art
It is necessary in numerous cases to have knowledge about the kind and concentration of gases present in the breathable air in order to be able to perform technical control processes and technical automatic control processes or in order to be able to perform steps for the protection of the human being relative to dangerous or unpleasant, smelling gases. The knowledge of the contents of damaging materials inside or outside of the living space of the human being is important in particular for the purpose of the necessary and situation-required ventilation, in order to either initiate the ventilation or in order to interrupt the feeding of outside air when concentrations of damaging gas are present outside of the region to be ventilated.
During the surveillance or automatic control of combustion processes it is important to know the concentration of non-combusted hydrocarbons or of carbon monoxide or, respectively, the concentration of nitrous oxides in the waste gas. It is necessary in the course of monitoring the function of catalysts, for example, in motor vehicles, to know the concentration of gases before and after the catalyst, in order to be able to determine the degree of effectiveness of the catalyst.
It is a property of all examples that both, oxidizable gases, hydrocarbons, carbon monoxide, etc., as well as of reducible gases, in particular nitrous oxides, are to be detected for a determination of the individual situation and for the control of processes or, respectively, for the initiation of proper steps.
Based on the massive requirement of such sensors, technologies are sought which are not associated with large financial costs. Sensors based on polycrystalline metal oxides are associated with favorable costs and are sturdy and have been proven in practical situations. However, at this time experiences exist in fact relative to the detection of oxidizable materials, but hardly any experiences or practical solutions exist relative to the detection of both oxidizable gases as well as reducible gases, which occur simultaneously and which are to be detected at the same time, i.e. simultaneously, without making the costs and the equipment requirements too expensive relative to the application.
In principle, metal oxide sensors change their electrical resistance upon presence of a gas. The generally known sensors consist of a heated and contacted layer made for example of tin dioxide or of another metal oxide such as, for example, zinc oxide, gallium oxide, tungsten trioxide, aluminum vanadate, and other sensitive materials, wherein the sensor material is applied in a thin film technique or a thick film technique to a substrate made of ceramics or silicon and exhibits contact electrodes. If an oxidizable gaseous material impinges onto the sensor, then the metal oxide releases oxygen and becomes therefore reduced, whereby the conductivity value is increased. This process is reversible because the heated metal oxide combines later again with the oxygen of the air. The conductivity value of the sensor material is a function of the concentration of the oxidizable gases present, since a continuous exchange process is present between the offered gas, the metal oxide and the oxygen of the air and because the concentration of the oxygen of the air is a quasi constant value.
The electrical effect is opposite in case of reducible gases such as ozone, nitrous oxides and the like, because the sensitive material of the sensor is in no case a pure metal oxide, but instead this metal oxide is always in part reduced [SnO (2xe2x88x92x)]. Thus, the conductivity value is therefore lowered in case of an application of gas to the sensor. Complex chemical reactions between the gases with the sensor material as well as of the gases amongst each other occur in the simultaneous presence of oxidizable gases and of reducible gases in the neighborhood of the hot surface of the sensor, furnished with catalytic materials such as for example platinum or palladium. An unequivocal statement relating to the concentration of the gases present cannot be made up to now in this case.
It has been observed that the reactivity of metal oxide sensors relative to oxidizable or reducible gases is dependent on temperature. In case of low temperatures of for example below 150xc2x0 C., the tin dioxide sensors are only slightly sensitive relative to oxidizable gases and hardly at all react with a conductivity-value increase upon application of a gas. If however reducible gases are present, the sensor reacts immediately with a significant decrease in the value of the electrical conductivity. For this reason it has been proposed to switch either sensor groups of in each case constant temperature based on the different reactions of the sensor in case of high (above 300xc2x0 C.) temperatures and of low (below 150xc2x0 C.) temperatures, which sensor group is in each case more sensitive for the group of gases involved.
It is known from the German printed patent document DE-A-38 27 426 to vary the temperature of the sensor and thus to obtain the desired information with a single sensor. This method has not proven to be of practical interest, because it takes a long time for the sensor to recover a standard value after applying the gas to the sensor at a low temperature. It has further being observed that the metal oxide sensors have under no circumstances an Ohmic resistance changeable depending on the gas present, as is frequently reported in a simplified fashion.
A method is known from the German printed patent document DE-A-3917853, where the impedance course of the sensor element is determined through the frequency of an alternating current and is employed for the identification of the gas present. J. Gutierrez describes the electrical equivalent circuit diagram of a tin dioxide sensor (FIG. 1) as a combination of resistors and capacitors and determines that all parameters are changed upon presence of a gas, in the literature citation xe2x80x9cSensors and Actuators, Volume 4, 1991, pp. 359 to 363xe2x80x9d.
An electrically reactive compound device has in addition become known from the U.S. Pat. No. 5,387,462, which compound device exhibits random and regular fields of microstructures, which are in part disposed within an enveloping layer, wherein each microstructure exhibits a structure similar to a hair crystal and possibly exhibits a cover layer, wherein the cover layer envelopes the hair-crystal-like structure. The compound device is electrically conducting and serves as a component of an electrical switching circuit, of an antenna, of a micro-electrode, as a reactive heating element, or as a multimode sensor, in order tQ prove a presence of vapors, gases or liquids. The change of the orientation of the hair-like crystals is employed for measuring a presence of the material to be detected.
Technical Object:
It is object of the present invention to furnish a method and an apparatus which is or, respectively, are capable to detect simultaneously both groups of gases, namely oxidizable gases as well as reducible gases with one and the same sensor based on metal sensors, such as in particular the known metal oxide sensors.
1. Purposes of the Invention
It is a purpose of the invention to furnish a separate statement relative to the change of the electrical path resistance and to the change of the electrical contact resistance such that as statement relative to the presence and to the concentration of oxidizable gases and/or reducible gases can be made based on a single sensor.
These and other objects and advantages of the present invention will be come evident from the description which follows.
2. Brief Description of the Invention
The solution of the object resides with respect to a method in that the sensor is passed through by an alternating current, which alternating current consist either out of at least two alternating currents of different frequency is or which alternating current is switched between at least two frequencies, wherein on the one hand the change of the capacities between the sensor material and the contact electrodes is evaluated as being typical for the presence of reducible gases and on the other hand the change of the capacitances within the mass of the sensor material (sensitive material) is evaluated as typical for the presence of oxidizable gases, and wherein the changes of the Ohmic resistance components of the sensor material are considered in connection with the changes depending on the gas.
The sensor can be a component of an oscillator circuit for the generation of two different frequencies, wherein the oscillation of the oscillator circuit is periodically changed by switching the frequency-determining components between at least two frequencies, wherein the respective frequencies are individually evaluated and are compared to the respective frequencies determined during operation of the sensor in standard air and frequency deviations are employed as a measure for the concentration of present gas groups, wherein high frequencies are always coordinated to reducible gases and wherein low frequencies are always coordinated to oxidizable gases. According to a further embodiment, the phase shifts caused by the sensor-internal capacities are determined at the individual frequencies, and the output signal is decomposed into an imaginary part and into a real part, which imaginary part and real part represent a measure for the presence and for the kind of gases relative to the values determined under standard air conditions. Furthermore, the temperature of the sensor can be switched simultaneously with the switching of the frequencies, wherein the lower frequency is coordinated to the higher temperature and wherein the higher frequency is coordinated to the lower temperature.
A device is characterized in that the sensor is passed through by alternating current, which alternating current consists of at least two alternating currents of different frequencies or which is switchable between at least two frequencies, wherein on the one hand the change of the capacities between the sensor material and the contact electrodes is evaluated as typical for the presence of reducible gases and on the other hand the change of the capacities within the mass of the sensor material (sensitive material) is evaluated as typical for the presence of oxidizable gases, and wherein the gas-dependent changes of the Ohmic resistance components of the sensor material are considered in context.
The advantage of the method and of the device for this purpose comprise that both gas groups, namely oxidizable gases as well as reducible gases can be detected simultaneously with one and the same sensor, such as a metal sensor, in particular the conventional metal oxide sensors, wherein the device can be produced under favorable cost conditions.
The invention is based on the observation that the reaction mechanisms of oxidizable gases or of reducible gases, which are capable of being electrically evaluated, are substantially different. In case of an offer of oxidizable gases, for example of carbon monoxide CO, the Ohmic conductivity value of the mass of the sensor material changes as such by the reduction of the material. Also, the transition capacitance at the crystal transitions (path resistance) within the material are changed in a significant way. In contrast, the transition capacitance at the contacts is changed hardly at all.
The sensor reacts differently when reducible gases, for example nitrogen oxide NO or nitrogen dioxide NO2 are absorbed at the surface of the sensor material. Based on the lower reactivity, a lower change of the electrical parameters of the sensor mass (path resistance) or, respectively, of the sensor material occurs than in the case of oxidizable gases. However, a substantial influencing of the Schottky transitions occurs caused by the gas-induced boundary face states.
If therefore a sensor is operated only with direct current as a gas-dependent resistor, then the switching capacitances at the Schottky transitions become part of the background. Practically only the mass resistance of the polycrystalline metal oxide is evaluated.
One obtains in contrast other and substantially richer results, if the impedance of the sensor is considered within the framework of evaluation strategies. The invention furnishes therefore a device which allows the simultaneous detection both of oxidizable substances and of reducible substances with one single sensor, wherein the changes of the above described capacitances are employed and exploited, which changes are a function of the nature of the gas.
The novel features which are considered as characteristic for the invention are set forth in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.